For industrial robots, especially the ones designed to work in limited and complex workspaces, flexibility and compactness are two major design requirements. Generally, the larger the rotation ranges of robot joints are, the more flexible the robot is. Usually, cables and hoses are routed within the robot joints to prevent interfering with surroundings. But these cables cannot bear too much bending and twisting for a long time. In addition, improper routing structures may cause cables and hoses bending in small radius, twisting in small length, or sliding on coarse surfaces, all of which will reduce the lifetime of cables and lead to unexpected shutdown.
One common solution is to use flexible printed circuit (FPC) boards inside the robot joints to transmit power and various signals. FIG. 1A shows the expanded view of a normal FPC board in a shape of a broken ring. FIG. 1B shows perspective view of the FPC board after bending. As shown in FIG. 1B, the FPC board is bent at a bending portion 30 to from a first portion 10 and a second portion 20. The first portion 10 is rotatable with respect to the second portion 20 around a rotation axis of the robot joint. Theoretically, the two portions 10 and 20 have an overlapping area of no larger than 180 degrees around the rotation centre, which means the rotation range of the FPC board is smaller than ±360 degrees. Actually, considering the bending radius and other factors, the overlapping area can only cover about 100 degrees as shown in FIG. 1C. Therefore, the rotation range of the FPC board is smaller than ±200 degrees, which is not even close to the ideal rotation range of ±360 degrees.